1. Field of the Invention
This invention relates to improvements in investment casting and in particular to the rapid removal of cores made of .beta.-Al.sub.2 O.sub.3 employed in the casting process.
2. Description of the Prior Art
The production of directionally solidified (DS) metal eutectic alloys and superalloys for high pressure turbine (HPT) airfoils with intricate internal passageways for air cooling requires that the core and mold not only be dimensionally stable and sufficiently strong to contain and shape the casting but also be sufficiently weak to prevent mechanical rupture (hot tearing) of the casting during solidification and cooling. The DS process requirements of up to 1875.degree. C. for a 16 hour time period imposes severe constraints on materials which may serve as mold or core candidates.
The currently available core materials do not possess the chemical stability required for casting eutectic alloy and superalloy materials. The prior art appears to be mostly limited to the use of silica or silica-zircon core and mold materials. At temperatures greater than 1600.degree. C. the silica based materials fail from the standpoint of both mechanical integrity and chemical incompatibility with the advanced alloy compositions.
Aluminum oxide (Al.sub.2 O.sub.3) by itself, without a chemical or physical binder material, has been identified as a potential core and mold material based on both chemical compatibility and leachability considerations.
In co-pending patent applications Ser. No. 775,762 now U.S. Pat. No. 4073662, and Ser. No. 839,990 filed Mar. 9, 1977 and Oct. 6, 1977, and assigned to the same assignee of this invention, alumina and alumina-based compound material compositions are disclosed for making improved cores for D.S. casting. In particular, the cores have a porosity of greater than about 20% by volume and have excellent crushability characteristics and are resistant to metal-core reaction. Depending upon processing techniques, the core may or may not have an integral dense alumina outer layer to prevent metal penetration. Cores made from the material compositions must be removed from the castings without any deleterious effect on the surface of the casting. However, Al.sub.2 O.sub.3 and other dense advanced core materials such as LaAlO.sub.3, MgAlO.sub.4 and Y.sub.3 Al.sub.5 O.sub.12 are not easily attacked by the conventional autoclave techniques used for SiO.sub.2.
In co-pending application Ser. No. 775,761, filed on Mar. 9, 1977, now abandoned, and assigned to the same assignee as this invention, autoclave leaching of magnesia doped alumina is taught and claimed therein. It is the belief that the addition of the divalent alkaline earth cations into the trivalent cation lattice of Al.sub.2 O.sub.3 introduces lattice defects which enhance the kinetics of the dissolution of alumina.
It is therefore an object of this invention to provide a new and improved method for removing .beta.-alumina core material from directionally solidified eutectic alloy and superalloy materials.
Another object of this invention is to provide a new and improved method for the rapid removal of porous .beta.-alumina core material by a caustic solution in an autoclave without adversely affecting the cast material.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.